A typical crankshaft has main bearings at its end, and in some cases in the middle that lie on and define a main crankshaft or workpiece axis. It also has a number of crankpins angularly equispaced about and from the main axis, for example six in two arrays of three. Each crankpin is therefore centered on an axis that extends parallel to but is radially offset from the main workpiece axis. Such a workpiece is cast and is then finished machined to very high tolerances. While the finished workpiece must meet very tight tolerances, such crankshafts are produced in large numbers and must be manufactured as rapidly and cost-efficiently as possible, which means that any manual handling must be minimized or eliminated.
Grinding the main bearings is relatively easy since the workpiece can be chucked at one end and rotated about its axis while grinding stones or other machining tools do the required finish work. For grinding the crankpins, the workpiece must rotated about the axis of the crankpin or crankpins being machined.
To this end the workpiece can be held in a pair of chucks such as described in DE 10 2006 011 057 that chucks having an outer part adapted to be rotated about an axis and a core part that can be shifted in a displacement direction radially of and orthogonal to this axis. Thus it is possible to hold a crankshaft between two such chucks and, by various manipulations, either orient the main-bearing axes or the crankpin axes concentric with the chuck axis, which here is also the so-called grinding axis on which the grinding tools are centered. Such a system does make it possible to machine a crankshaft without having to completely dechuck and rechuck it for each grinding operation, it still entails a number of complex manual setup operations.